Flow-through sterilizers of the type used in hospitals or in industrial applications are generally provided with access to a source of saturated steam "on tap" which is used to purge air from a chamber containing the articles to be sterilized and to heat the load until sterilization exposure is complete.
In the usual dental or medical office, it would be physically and economically impractical to have an available steam line, and so sterilization of instruments in these applications has hitherto been carried out using standard autoclaves. An autoclave is typically a relatively large sealed vessel which contains a quantity of boiling water under pressure. Such an instrument is generally provided with a lower exit vent which is initially kept open for a period while water is boiled inside the autoclave to purge the air from the interior. As air is incompatible with and denser than saturated steam, it tends to be pushed out the exit valve ahead of the steam.
Nevertheless, the standard arrangement can lead to appreciable quantities of air being retained in the autoclave chamber, necessitating the simultaneous measurement of both pressure and temperature conditions in the chamber if one is to have confidence in the sterilizing conditions to which the contents are exposed. Imperfect purging of air or an incorrect interpretation of the simultaneous readings of the mutually dependant variables of temperature and pressure could present a biological hazard. On the other hand, unduly high temperatures in the standard autoclave can, with time, damage the contents.
Moreover, in a closed system such as the standard autoclave, or in other known systems which employ a circulating water phase, contaminants from the instruments constantly build up with time, leading to corrosion of the autoclave walls, and possible damage to the contents. A further practical disadvantage of standard autoclaves is their inherent slowness, due to the large heat capacity of their heavy walls and support structures.